“Suppose my younger brother is playing with magnets near my Dell laptop. Can a magnet wipe data from the hard-drive or otherwise irreversibly damage it?”

Answer is:

“While you can damage hard-drives with magnets, it takes a very, very powerful magnet to do any damage. Any magnet that your brother may be playing with will not be large enough to do any damage:

Article Quote: This myth was popularized by movies where hackers or criminals would quickly erase the contents of their hard disk drives with a few sweeps of a powerful magnet. Unfortunately, it is almost impossible to do it with regular magnets, no matter how big they are.

Every hard disk drive actually contains two powerful neodymium-iron-boron magnets that control the movements of the read/write heads. Yet the data on the platters remain unaffected. It will take a very, very powerful magnet to affect the data inside the hard disk drive.”